Subsea power distribution system and method

Abstract

A power distribution system (100) is disclosed comprising: a topside power source (110); a subsea power distribution unit (120); at least one subsea load (130) connected to the power distribution unit (120); and a coaxial cable (140) configured to feed electric power from the topside power source to the subsea power distribution unit for powering the at least one load The topside power source is configured to feed the coaxial cable with an alternating current (AC) having a predetermined RMS current value. A sea-based system (200) comprising such a power distribution system is also disclosed, as is a related method.

Description

[0001] SUBSEA POWER DISTRIBUTION SYSTEM AND METHOD

[0002] Technical Field

[0003] The present disclosure and invention relates to a subsea power distribution system and to a related method.

[0004] In particular, the present disclosure and invention relates to subsea power distribution system configured to distribute electric power form a topside power source to a subsea power distribution unit connected to one or a plurality of subsea loads powered by the topside power source.

[0005] The present disclosure and invention also relates to a subsea hydrocarbon production and / or processing system comprising a subsea power distribution system.

[0006] Background

[0007] In subsea hydrocarbon production and / or processing systems, e.g. oil and gas production systems, it is known to use Direct Current Fibre Optic (DCFO or DC / FO) cable networks for distributing electric power to and transferring data to and from subsea apparatuses. Such systems are also used in submarine telecom systems. An example of a subsea oil and / or gas production system being power-supplied by a DC / FO cable network is disclosed in GB2497841B.

[0008] As is known in the art, a DC / FO cable usually has a coaxial structure and comprises an axial fibre unit comprising one or a plurality optical fibres, a first conductor surrounding the fibre unit, a first insulation sheet surrounding the first conductor, a second conductor surrounding the first insulation sheet, and a second insulation sheet surrounding the second conductor. The DC / FO cable may also be provided with one or a plurality of protective layers surrounding the second insulation sheet. DC / FO cables are typically classified as Light Weight (LW), Light Weight Protected (LWP), Single Armour (SA), and Double Armour (DA) depending on the number and type of protective layers surrounding the second insulation sheet.

[0009] Outgoing current may be conducted through the first conductor and return current through the second conductor. However, in some DC / FO cable systems the DC / FO cable may have only one conductor in operation - used for the outgoing current - and the return current may be conducted via the seawater column.

[0010] In subsea hydrocarbon production and processing systems powered by a DC / FO cable network, e.g. when control modules are powered by a DC / FO cable, electric power is typically provided as High Voltage Direct Current (HVDC), e.g. 10,000 VDC. An advantage of using Direct Current (DC) instead of Alternating Current (AC) is that transmission losses per unit distance can be lower using DC.

[0011] A DC / FO cable network applied in a subsea hydrocarbon production and / or processing system typically comprises subsea power distribution units, or hubs, configured to receive HVDC from a topside power source or power provider, e.g. being positioned on a host production platform or being land-based. The subsea power distribution units are further configured to distribute lower voltage DC to subsea loads, i.e. to the various apparatuses and sub-systems in the subsea hydrocarbon production and / or processing system. Consequently, a function of the subsea power distribution units is to convert HVDC received directly or indirectly from a DC / FO riser or landfall cable to a lower voltage level usable by the subsea loads. In the subsea power distribution units, the voltage is typically converted from around 10,000V DC (received from the topside power source via the DC / FO cable) to below 1,000V DC, e.g. to 400V DV (distributed to the subsea loads).

[0012] An advantage of using a DC / FO cable network in a subsea hydrocarbon production and / or processing system is that subsea DC / FO cables and accessories, e.g. connectors, cable joint boxes, cable end boxes, Y-splice boxes etc., are well-proven and readily available since DC / FO cable networks have been used in subsea data transmission networks for a long time.

[0013] However, one disadvantage of using a DC / FO cable network in a subsea hydrocarbon production and / or processing setting is that the inherent complexity of the DC / DC downconverters in the subsea power distribution units make such networks complex and expensive.

[0014] An object of the present disclosure and invention is to address this problem.

[0015] A further object of the present disclosure and invention is to provide a novel subsea power distribution system.

[0016] Yet a further object of the present disclosure and invention is to provide a new use of a coaxial cable, e.g. a DC / FO cable.

[0017] Another object of the present disclosure and invention is to extend the transmission distance at which AC is abandoned for DC due to AC source complexity.

[0018] Summary

[0019] It is generally held within the art of subsea power distribution that distribution AC electric power using a coaxial cable is not practically feasibly due to prohibiting losses. The present disclosure goes against this technical prejudice. In general, efficiency is very important for high power distribution systems. However, according to the aspects of the present disclosure, system efficiency may be sacrificed to an acceptable level allowing AC power to be distributed via coaxial cables and, thus, allowing subsea power distribution operators to benefit from the well-proven accessories (connectors, cable joint boxes, cable end boxes, Y- splice boxes etc.) available for coaxial cables, in particular the accessories available for DC / FO cables.

[0020] According to a first aspect, the present disclosure provides a power distribution system comprising: a topside power source; a subsea power distribution unit; at least one subsea load connected to the power distribution unit; and

[0021] - a coaxial cable configured to feed electric power from the topside power source to the subsea power distribution unit for powering the at least one load, wherein the topside power source is configured to feed the coaxial cable with an alternating current (AC) having a predetermined RMS current value. To utilise the capacity of the coaxial cable, the predetermined RMS current value may be equal to or less than the rated direct current (DC) value of the coaxial cable. In other words, the reactive cable current and the active cable current should not exceed the rated current of the cable at any position along the cable. Rated current may be linked to ambient temperature and lay-out configurations. Consequently, local cooling of the cable, e.g. when it runs above sea level in the vicinity of the topside power source, may be required if ambient temperature is high.

[0022] The predetermined RMS current value may be constant or varied, e.g. depending on the degree of loading of the load(s).

[0023] The predetermined RMS current value may be equal to or less than a rated direct current (DC) value of the coaxial cable. For example, the predetermined RMS current value may be within the range of 50%-100% of the rated direct current (DC) value of the coaxial cable.

[0024] The coaxial cable extends between and is electrically connected to the topside power source and the subsea power distribution unit, and the topside power source is configured to feed the coaxial cable with an alternating voltage (VAC) sufficient to power the load / loads. By maintaining the RMS current value at the rated direct current value of the coaxial cable and keeping the alternating feeding voltage as low as possible, reactive cable losses can be minimized.

[0025] The coaxial cable may comprise a first electric conductor and a first insulator sheet (dielectric) surrounding the first conductor. The coaxial cable may further comprise a second electric conductor surrounding the first insulator sheet, and a second insulator sheet (dielectric) surrounding the second conductor. The first and second conductors may typically share a common geometric axis.

[0026] The coaxial cable may comprise a one or a plurality of optical fibres. The optical fibre(s) may be axially aligned, e.g. surrounded by the first electrical conductor.

[0027] The coaxial cable may also comprise one or a plurality of protective or armouring layers arranged outside of the outer insulator sheet, i.e. outside of the first insulator sheet if the coaxial cable has only one conductor or outside of the second insulator sheet if the coaxial cable has two conductors.

[0028] The coaxial cable may be a Direct Current Fibre Optic (DC / FO) cable, e.g. a coaxial cable comprising one or a plurality of optical fibres (which typically may be axially aligned), a first electric conductor surrounding the optical fibre(s), a first insulator sheet (dielectric) surrounding the first conductor, a second electric conductor surrounding the first insulator sheet, and a second insulator sheet (dielectric) surrounding the second conductor, wherein the first and second conductors typically share a common geometric axis.

[0029] For distributing electric power from the topside power source to the subsea distribution unit, a first terminal of the topside power source may be electrically connected to the first conductor and a second terminal of the topside power source may be electrically connected to the second conductor. Likewise, a first terminal of the subsea power distribution unit may be electrically connected to the first conductor and a second terminal of the subsea power distribution unit may be electrically connected to the second conductor.

[0030] Alternatively, the first terminal of the topside power source and the first terminal of the subsea power distribution unit may be connected to the first conductor (and, if the coaxial cable has a second conductor, possibly also to the second conductor), and the second terminal of the topside power source and the second terminal of the subsea power distribution unit may be electrically connected to the sea / water column, thus allowing a path for the current between the second terminal of the topside power source and the second terminal of the subsea power distribution unit via the sea / water column.

[0031] From the subsea power distribution unit, electric power is distributed further to the subsea load / loads.

[0032] If the subsea load / loads are DC loads, the subsea power distribution unit may comprise an AC-to-DC power converter arrangement or power supply configured to convert AC voltage received from the top power source to a DC voltage level suitable for consumption by the subsea load / loads, e.g. 24 VDC. A transformer may be implemented between the subsea end of the coaxial cable and the input of the AC-to-DC power supply to transform the cable end voltage down to a range which is acceptable to the AC-to-DC power supply. This range could typically be 230 VAC + / - 30% or even wider.

[0033] If the first and second conductor is connected to first and second terminal, respectively, of the topside power source, the conductors in the coaxial cable act as a capacitor and, consequently, when alternating voltage is applied, the capacitance properties appear in parallel with the electrical load. This leads to heat losses in the coaxial cable due to reactive current. By keeping the cable current provided by the topside power source constant at the rated value and the alternating input voltage as low as possible, the reactive cable losses are minimized.

[0034] The output impedance of the coaxial cable (including the output or source impedance of the topside power source, i.e. the internal impedance of the topside power source), and the input impedance of the subsea power distribution unit (including the input impedance of the subsea load / loads) should preferably be matched in order to provide maximum power transfer from the topside power source to the subsea power distribution unit. Additionally, or alternatively, the input impedance of the coaxial cable may be matched to the output impedance of the topside power source.

[0035] By arranging an inductive shunt and / or serial capacitor elements at the cable junction between the topside power source and the coaxial cable junction box the reactive cable current may be mitigated or neutralized. Such matching may alternatively or additionally be implemented using electronic phase compensation at the topside power source in the same way as in conventional AC power systems.

[0036] Alternatively, or additionally, losses in the coaxial cable may be reduced by reducing the frequency of the AC topside power source using a frequency converter. In this regard, a topside power source operating at 50Hz is preferred over one operating at 60Hz.

[0037] The topside power source may be configured to adjust the output voltage to the input impedance of the at least one subsea load to maximise the transmitted power. In other words, the topside power source may be configured to adjust the output voltage to bring the output impedance of the coaxial cable (including the output or source impedance of the topside power source) to match the input impedance of the subsea power distribution unit and the at least one subsea load connected thereto. This may involve implementing a maximum power point tracking regime by measuring or estimating the input impedance(s) of the load(s) and adjusting the output voltage of the topside power source to maximise power transmission. The subsea power distribution unit may be configured to collect information on the input impedance of the subsea load / loads and forward this information to the topside power source for the adjustment of the output voltage. The subsea power distribution unit may be configured to measure the input current and voltage of subsea load / loads and calculating the input impedance of the subsea load / loads based on the measured input current and voltage values. If the coaxial cable is a DC / FO cable, the optical fibres in the DC / FO cable may be used for transmitting the information on the input impedance of the load / loads from the subsea power distribution unit to the topside power source.

[0038] Alternatively, or additionally, the input impedance of the at least one subsea load may be at least partially estimated based on an operating state of the subsea load / loads. Thus, the input impedance need not necessarily be measured. For example, in a subsea hydrocarbon production and / or processing environment, the input impedance of different loads, e.g. valve actuators etc., at different operating states may be known. Similarly, the topside power source including the cable and the rest of the electrical system may be electrically modelled. Consequently, upon commands being provided to the load / loads (powered by the topside power source via the subsea power distribution unit) to assume different operation states, the topside power source can be instructed to provide, at any point in time, an output voltage that, given the constant RMS current value at which the topside power source operates, maximises the power transfer. The output current and the output voltage of the topside power source may be controlled by a microprocessor, e.g. implementing a numerical electrical network model that tracks the input impedance of the subsea load / loads.

[0039] According to a second aspect, the present disclosure provides a sea-based system comprising a power distribution system according to the first aspect for distributing electric power from a topside location to a subsea location of the sea-based system.

[0040] The sea-based system may be any one of: a hydrocarbon production and / or processing system; a subsea carbon capture and storage (CCS) system; and a power production and / or power distribution system. If the sea-based system is a hydrocarbon production and / or processing system, the topside location may be a hydrocarbon production and / or processing platform and the subsea location may be a subsea hydrocarbon production and / or processing field comprising hydrocarbon subsea wells and associated infrastructure. The sea-based system may be any one of: a sea-based windfarm; a wave power system; and a tidal power system.

[0041] According to a third aspect, the present disclosure provides a method of distributing electric power from a topside power source to a subsea power distribution unit connected to at least one subsea load in a sea-based system, e.g. any one of: a hydrocarbon production and / or processing system; a subsea carbon capture and storage (CCS) system; and a power production and / or power distribution system, e.g. a sea-based windfarm, a wave or a tidal power system. The method comprises the step of feeding a coaxial cable running from the topside power source to the subsea power distribution unit with an alternating current (AC) having a predetermined RMS current value. The predetermined RMS current value may be the rated direct current (DC) value of the coaxial cable.

[0042] The topside power source impedance may be controlled by adjusting the voltage of the topside power source (keeping the current at the predetermined RMS current value). To maximise transmitted electric power the method may comprise the step of adjusting the output voltage of the topside power source such that the output impedance of the coaxial cable (including the output or source impedance of the topside power source) is matched to the input impedance of the subsea power distribution unit and the at least one subsea load connected thereto.

[0043] The method may further comprise the step of collecting information on the input impedance of the at least one subsea load and forwarding this information to the topside power source for said adjustment of the output voltage, thereby enabling the topside power source to provide an output voltage such that a desired relationship between the voltage and the current at the subsea load is achieved.

[0044] The method may also comprise the step of estimating or calculating the input impedance of the at least one subsea load based on an operating state of the at least one subsea load. According to a fourth aspect, the present disclosure provides a use of a coaxial cable for distributing topside AC power from a topside power source to a subsea power distribution unit connected to at least one subsea load in a sea-based system, e.g. any one of: a hydrocarbon production and / or processing system; a subsea carbon capture and storage (CCS) system; and a power production and / or power distribution system, e.g. a sea-based windfarm, a wave power system or a tidal power system. Said use may involve distributing the topside AC power as an alternating current (AC) having a predetermined RMS current value. The predetermined RMS current value may be the rated direct current (DC) value of the coaxial cable.

[0045] Above-discussed required, preferred and / or optional features of each aspect of the disclosure may be used, alone or in appropriate combination, in the other aspects of the disclosure.

[0046] The claimed invention is specified in the independent claims of this application. Advantageous adaptations and versions of the claimed invention are specified in the dependent claims.

[0047] Description of the drawings

[0048] Following drawings are appended to facilitate the understanding of the claimed invention:

[0049] Fig. 1 schematically illustrates a hydrocarbon production and / or processing system comprising a power distribution system;

[0050] Fig. 2 schematically illustrates matching impedance.

[0051] It should be understood, however, that the drawings are not intended to limit the claimed invention to the subject-matter depicted in the drawings.

[0052] In the drawings, like reference numerals have been used to indicate common parts, elements or features unless otherwise explicitly stated or implicitly understood by the context.

[0053] Detailed description

[0054] In the following, one or more specific embodiments of the invention will be described in more detail with reference to the drawings. However, it is specifically intended that the claimed invention is not limited to the embodiments and illustrations contained herein but includes modified forms of the embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation- specific decisions must be made to achieve the developer’s specific goals, such as compliance with system and / or business-related constraints, which may vary from one implementation of the invention to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication and manufacture for the skilled person having the benefit of this disclosure. Fig. 1 schematically illustrates an embodiment of a hydrocarbon production and / or processing system 200 comprising a power distribution system 100 according to one embodiment of the present invention. The power distribution system 100 comprises a topside power source 110, a subsea power distribution unit 120, at least one subsea load 130 connected to the power distribution unit 120, and a coaxial cable 140 configured to feed electric power from the topside power source 110 to the subsea power distribution unit 120 for powering the at least one load / loads 130. Consequently, the coaxial cable 140 extends between and is electrically connected to the topside power source 110 and the subsea power distribution unit 120.

[0055] The coaxial cable 140 comprises a first conductor 141, a first insulator sheet (dielectric) 142 arranged coaxially outside of the first conductor 141, a second conductor 143 arranged coaxially outside of the first insulator sheet 142, and a second insulator sheet 144 arranged coaxially outside of the second conductor 143. The coaxial cable 140 may also comprise one or a plurality of armouring layers 145.

[0056] The coaxial cable may be a Direct Current Fibre Optic (DC / FO) cable, i.e. a coaxial cable comprising one or a plurality of optical fibres 146 around which the first conductor 141 is coaxially arranged.

[0057] The topside power source 110 is configured to feed the coaxial cable 140 with an alternating current (AC) having a predetermined RMS current value. To utilise the capacity of the coaxial cable 140, the predetermined RMS current value fed into the coaxial cable 140 from the topside power source 110 may correspond to a rated direct current (DC) value of the coaxial cable.

[0058] The topside power source 110 is configured to feed the coaxial cable 140 with a voltage (VAC) sufficient to power the load / loads 130 at any point in time. By keeping the cable current constant at the rated RMS current value and the alternating feeding voltage as low as possible, reactive cable losses are minimized.

[0059] Preferably, the output impedance of the coaxial cable 140 (including the output or source impedance of the topside power source 110, i.e. the internal impedance of the topside power source 110) and the input impedance of the subsea power distribution unit 120 (including the input impedance of the subsea load / loads 130) are matched to provide maximum power transfer from the topside power source 110 to the subsea power distribution unit 120.

[0060] In addition, or alternatively, and in accordance with the maximum power transfer theorem, the input impedance of the coaxial cable 140 (the load impedance) should preferably be equal to the complex conjugate of the output impedance of the topside power source 110 (the source impedance).

[0061] Such matching may be implemented by arranging a matching impedance 150 at the cable junction between the topside power source and the coaxial cable junction box, as is schematically illustrated in Fig. 2, in which figure the coaxial cable 140 is modelled by resistors, inductors and a capacitor in a T-configuration, and the load / loads are represented by a resistor. The matching impedance 150 is realised by an inductive and a resistive shunt 151, 152. In Fig. 1 a matching impedance shunt is schematically illustrated as 150. In some applications, it may be advantageous for the impedance matching to add at least one matching impedance along the DC / FO cable. This can be realized in a DC / FO cable field splice or T or as part of an intermittent distribution unit.

[0062] Alternatively, or additionally, the matching impedance may comprise serial capacitor elements at the cable junction between the topside power source and the coaxial cable junction box.

[0063] Such matching may alternatively or additionally be implemented using electronic phase compensation at the topside power source 110 in the same way as in conventional AC systems.

[0064] Alternatively, or additionally, losses in the coaxial cable may be reduced by reducing the frequency of the AC topside power source using a frequency converter.

[0065] As previously stated, power distribution system 100 uses AC constant current fed from the top-side. This is in contrast to conventional umbilical based AC power distribution systems which usually uses constant voltage feeding.

[0066] The topside power source 110 may advantageously be configured to adjust the output voltage to the input impedance of the subsea loads 130 to maximise the distributed power. This may involve implementing a maximum power point tracking regime by measuring or estimating the input impedances of the subsea loads 130 and adjusting the output voltage of the topside power source 110 to maximise power transmission. The subsea power distribution unit 120 may be configured to collect information on the input impedance of the subsea loads 130 and forward this information to the topside power source 110 for the adjustment of the output voltage. The subsea power distribution unit 120 may be configured to measure the input current and voltage of the subsea loads 130 and calculate the input impedance of the subsea loads based on the measured input current and voltage values. If the coaxial cable is a DC / FO cable, the optical fibre(s) 146 in the DC / FO cable may be used to transmit the input impedance information from the subsea power distribution unit 120 to the topside power source 110, as is indicated by the dashed lines in Fig. 1.

[0067] According to one embodiment, the input impedance of the subsea loads 130 may be at least partially estimated or calculated based on the operating state of the subsea loads 130. Thus, the input impedance need not necessarily be measured for each subsea load 130. For example, in a subsea hydrocarbon production and / or processing environment, the input impedance of different loads, e.g. valve actuators etc., at different operating states may be known. Consequently, upon commands being provided to the load / loads 130 to assume different operation states, the topside power source can be instructed to provide, at any point in time, an output voltage that, given the constant RMS current value at which the topside power source operates, maximises the power transfer. The output current and the output voltage of the topside power source 110 may be controlled by a microprocessor 111, e.g. implementing a numerical network model that tracks the input impedance of the subsea loads.

[0068] In the preceding description, various aspects of the apparatus according to the invention have been described with reference to the illustrative embodiment. For purposes of explanation, specific numbers, systems and configurations were set forth in order to provide a thorough understanding of the apparatus and its workings. However, this description is not intended to be construed in a limiting sense. Various modifications and variations of the illustrative embodiment, as well as other embodiments of the apparatus, which are apparent to person skilled in the art to which the disclosed subject-matter pertains, are deemed to lie within the scope of the present invention as defined by the following claims. For example, it is understood that the biasing means configured for biasing the safety valve towards the failsafe position may not necessarily be a spring. In other embodiments the biasing means may comprise a hydraulic pressure source hydraulically biasing the safety valve stem. The hydraulic pressure source may for example comprise a hydraulic reservoir or an accumulator. In other embodiments still, the hydraulic pressure source may comprise process or production fluid. In other words, necessary hydraulic pressure for biasing the safety valve stem towards the fail-safe position may be received from process or production fluid in the subsea hydrocarbon production or processing system.

Claims

Claims1. A power distribution system (100) comprising:- a topside power source (110);- a subsea power distribution unit (120);- at least one subsea load (130) connected to the power distribution unit (120); and- a coaxial cable (140) configured to feed electric power from the topside power source (110) to the subsea power distribution unit (120) for powering the at least one load (130), characterised by the topside power source (110) being configured to feed the coaxial cable (1 0) with an alternating current (AC) having a predetermined RMS current value.

2. The power distribution system (100) according to claim 1, wherein the predetermined RMS current value is equal to or less than a rated direct current (DC) value of the coaxial cable (140).

3. The power distribution system (100) according to claim 2, wherein the predetermined RMS current value is within the range of 50%-100% of the rated direct current (DC) value of the coaxial cable (140).

4. The power distribution system (100) according to any one of the preceding claims, wherein the coaxial cable (140) comprises a first electric conductor (141) and a first insulator sheet (142) surrounding the first conductor (141).

5. The power distribution system (100) according to claim 4, wherein the coaxial cable (140) comprises a second electric conductor (143) surrounding the first insulator sheet (142), and a second insulator sheet (144) surrounding the second conductor (143).

6. The power distribution system (100) according to any one of claims 4 and 5, wherein the coaxial cable (140) comprises one or a plurality of optical fibres (146).

7. The power distribution system (100) according to any one of the preceding claims, wherein the output impedance of the coaxial cable (140) and the input impedance of the subsea power distribution unit (120) are matched.

8. The power distribution system (100) according to any one of the preceding claims, wherein the topside power source (110) is configured to adjust the output voltage to bring the output impedance of the coaxial cable (140) to match the input impedance of the subsea power distribution unit (120) and the at least one subsea load (130) connected thereto.

9. The power distribution system (100) according to claim 8, wherein at least one of the subsea power distribution unit (120) and the at least one subsea load (130) is configured to collect information on the input impedance of the at least one subsea load (130) and forward this information to the topside power source (110) for said adjustment of the output voltage.

10. The power distribution system (100) according to any one of claims 8 and 9, wherein the input impedance of the at least one subsea load (130) is at least partially estimated or calculated based on an operating state of the at least one subsea load (130).

11. A sea-based system (200) comprising a power distribution system (100) according to any one of the preceding claims for distribution electric power from a topside location to a subsea location of the sea-based system.

12. The sea-based system (200) according to claim 11, wherein the sea-based system (200) is any one of: a hydrocarbon production and / or processing system; a subsea carbon capture and storage (CCS) system; and a power production and / or power distribution system.

13. The sea-based system (200) according to claim 12, wherein the power production and / or power distribution system is any one of: a sea-based windfarm; a wave power system; and a tidal power system.

14. A method of distributing electric power from a topside power source (110) to a subsea power distribution unit (120) connected to at least one subsea load (130) in a sea-based system (200), comprising the step of:- feeding a coaxial cable (140) running from the topside power source (110) to the subsea power distribution unit (120) with an alternating current (AC) having a predetermined RMS current value.

15. The method according to according claim 14, comprising the step of adjusting an output voltage of the topside power source (110) such that an output impedance of the coaxialcable (140) is matched to an input impedance of the subsea power distribution unit (120) and the at least one subsea load (130) connected thereto.

16. The method according to according to claim 15, comprising the step of:- collecting information on the input impedance of the at least one subsea load (130) and forwarding this information to the topside power source (110) for said adjustment of the output voltage.

17. The method according to according any one of claims 15 and 16, comprising the step of:- estimating or calculating the input impedance of the at least one subsea load (130) based on an operating state of the at least one subsea load (130).

18. Use of a coaxial cable (140) for distributing topside AC power from a topside power source (110) to a subsea power distribution unit (120) connected to at least one subsea load (130) in a sea-based system (200).

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